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Tiny Tarbosaurus Shows How Tyrants Grew Up

The new Tarbosaurus juvenile is a truly remarkable specimen

In 2006, paleontologists searching the western Gobi Desert under the auspices of the Hayashibara Museum of Natural Sciences–Mongolian Paleontological Center Joint Expedition uncovered a rare fossil prize—the nearly complete skeleton of a juvenile Tarbosaurus bataar. The closest relative to Tyrannosaurus rex, Tarbosaurus was just as big and bad as its North American counterpart, but until now, how this tyrant dinosaur grew up had mostly been inferred from what is known from other dinosaurs. Not only does the new specimen allow paleontologists to better estimate how Tarbosaurus changed as they aged, but it also raises questions about the identity of other young tyrants that have been the focus of long-running debates over dinosaur lives.

For paleontologists Takanobu Tsuihiji, Mahito Watabe and colleagues, the identity of the young Tarbosaurus was fairly easy to determine. Tarbosaurus is the only large tyrannosaur found at the dinosaur-rich Bugin Tsav fossil site, and the only other tyrannosaur found in the same age range between about 70 million and 65 million years ago—Alioramus—was anatomically very different. (Zhuchengtyrannus, another large tyrannosaur found in China and announced last month, came from a different, distant, and slightly older site.) In particular, the new specimen had 13 teeth in the main portion of the upper jaw (the maxilla) paired with 14 and 15 teeth in each side of the lower jaw, which falls within the range of variation seen in adult Tarbosaurus. While this may seem to be a trivial feature, the toothcounts of tyrannosaurs is one of the ways paleontologists distinguish between species and, sometimes, between adults and juveniles of the same species.

In addition to the slenderness of the skull, the large opening for the eye, and other tell-tale features, the juvenile status of the recently discovered Tarbosaurus was confirmed through the microstructure of its leg bones. The observable growth lines inside the leg bones pinned the individual at 2 to 3 years old. This Tarbosaurus was a pre-teen—it perished just before the big growth spurt observed in the growth patterns of North American tyrannosaurs. Furthermore, the nearly complete new specimen is very similar to another partial skeleton given the name “Shanshanosaurus huoyanshanensis” in 1977, which itself is likely to be another Tarbosaurus of a similar age. Thanks to the completeness of the new specimen, another of China’s many enigmatic partial dinosaur skeletons can finally be matched to the proper species.

Curiously, though, the skull of the young Tarbosaurus hints that juvenile tyrants selected from a different menu than the adults. As has been seen among other young tyrannosaurs, the skull of the young Tarbosaurus is relatively shallow and slender, fitted with teeth that are more like knives than serrated railroad spikes as in adults. With these traits, the authors of the new Journal of Vertebrate Paleontology study suggest, juvenile Tarbosaurus was incapable of delivering the heavy crushing bites of the adults and therefore may have specialized on shearing flesh from small prey. Likewise, the young Tarbosaurus had a large ring of bone in its eye—called the sclerotic ring—that would have supported a big eye. Based on a study of how these structures relate to nocturnal activity in dinosaurs, juvenile Tarbosaurus may have hunted in low-light conditions while their parents stalked bigger game during the day or at some other time. If correct, this means that juvenile Tarbosaurus did not compete directly with adults for food in the same habitats—a phenomenon called niche partitioning.

But the new find does more than inform our understanding of Tarbosaurus growth. The new specimen also provides a little more context for at least two ongoing arguments about other tyrannosaur specimens.

Over the past several decades paleontologists have been debating the identity of several dinosaur specimens sometimes said to belong to a genus of “pygmy tyrant” called “Nanotyrannus.” Depending on who you ask, the specimens attributed to this hypothetical genus are either juveniles of Tyrannosaurus or belong to a distinct species, and the new Tarbosaurus has some bearing on this discussion.

Whereas the young Tarbosaurus had the same number of teeth as adult specimens, purported “Nanotyrannus”/juvenile Tyrannosaurus skulls often have several more teeth than adult Tyrannosaurus. A change in tooth shape has been cited as the reason for the difference—as Tyrannosaurus grew up, their teeth became more robust and a few teeth were lost to make room—but the corresponding tooth counts in the closely related Tarbosaurus complicates the picture. The change in tooth counts between the adult and juvenile specimens in Tyrannosaurus may represent a truly unique growth pattern, or may be expressions of variation between individuals rather than changes in growth. (That is, if the “Nanotyrannus” specimens truly are juvenile Tyrannosaurus.) At present, the disparity is something that requires further study of multiple tyrannosaur growth series to resolve.

The second area of debate involves the recently-described tyrannosaur Raptorex. When this dinosaur was described in 2009, it was presented as evidence that many characteristic traits of the largest tyrannosaurs evolved at a small body size. Yet, because of its small size and estimated age of 5 to 6 years old, there is a possibility that the Raptorex specimen is truly the juvenile of another tyrannosaur species. (This idea was broached by a NatureNews article released during last year’s Society of Vertebrate Paleontology meeting, though there was no actual study supporting the hypothesis at the time.) Juvenile dinosaurs often retain archaic traits that cause them to resemble other species that fall out towards the base of their family tree. This makes it difficult to tell the difference between a juvenile and a small adult of a more basal species. Figuring out which category Raptorex belongs in will require more research, though the authors of the new paper report that this other small tyrant specimen was not a juvenile Tarbosaurus (contrary to what was suggested in the NatureNews report). The Tarbosaurus juvenile and the Raptorex specimen differ in at least one major aspect of the hip and several skull characteristics, and this—added to the separation between them in space and time—means that if Raptorex is a juvenile, then it is a juvenile of some other tyrannosaur species.

The new Tarbosaurus juvenile is a truly remarkable specimen. Not only does the skeleton provide paleontologists with a relatively complete look at a moment in the growth of Tarbosaurus, but the bones may also reinvigorate long-running debates about tyrannosaur growth. Young tyrannosaurus were not just miniature copies of adults—the large tyrannosaurus of the late Cretaceous underwent some major changes as they grew—but parsing the fine line between juveniles of giant genera and distinct species of pygmy tyrants remains a contentious area of research.

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